(1) Field of the Invention
The present invention relates to an image forming apparatus using electrophotography such as a copier, printer, facsimile machine or the like, in particular, relating to a tandem type image forming apparatus.
(2) Description of the Prior Art
In conventional color printers, color multi-functional machines and the like, when a monochrome mode printing (including text printing) is implemented, usually no color image forming for colors (C/M/Y) is carried out, hence the photoreceptors and developing units for colors are kept from driving without applying any high voltage to the developing units while the photoreceptor and developing unit for black (BK) alone are driven. These image forming apparatus often employ a configuration in which two or more number of development transformers for image forming are used. Because the development transformer including the control circuit for controlling the transformer is markedly expensive, provision of such a development transformer for each developing unit leads to sharp increase of the machine cost.
When monochrome printing is selected in a system using an intermediate transfer element, it is necessary to perform control so as to stop not only the photoreceptors and developing units for colors but also to move and kept the intermediate transfer element away from the photoreceptors for colors or the like.
In view of cut down the cost of the machine, there has been devised a configuration in which one common development transformer is shared by all the developing units while high voltage is adapted to be applied also to C/M/Y color developing units even in a monochrome mode. Alternatively, patent document 1 (Japanese Patent Application Laid-open H5-197254) discloses an image forming apparatus of a fixed developing unit system in which a single developing bias power source is used in common so as to supply power only to the developing units that are activated by a switching means.
In the above way, according to the aforementioned conventional image forming apparatus, miniaturization and cost down of the machine are achieved by enabling the apparatus to operate with only a single development transformer, which is high in price.
However, in the configuration in which a single developing transformer is shared so as to constantly apply high voltage to each developing unit, there is the problem that when the photoreceptors for colors are stopped to drive, there occurs electric damage to each of the photoreceptors opposing the color developing units, producing defects on their surfaces.
Also, in patent document 1 (Japanese Patent Application Laid-open H5-197254), high accuracy of control timing is demanded in switching control of the developing bias voltage from the developing bias power source, hence the switching control circuit cannot but become complicated.